1. Field of the Invention
The present invention relates to a color image capturing device used for an image reader and other similar devices.
2. Description of the Related Art
Image readers for moving a carriage supporting a color image capturing device, and for reading an image on a manuscript, are known. The image readers include multiple image sensing elements, which are aligned with each other on a color image capturing device, and three line sensors for respectively reading three primary colors, such as a CCD in parallel with the surface of a manuscript.
For example, in the case of a flat-bed image reader, a manuscript table, composed of a glass transparent plate for placing a manuscript on the upper surface of a box cabinet, is provided and inside the cabinet, a carriage moved in parallel with the manuscript table by a drive unit is provided. A light source and the color image capturing device are mounted on the carriage. Illuminating light from the light source is reflected by the surface of a manuscript on the manuscript table and focused on the color image capturing device by a condenser lens.
In the above image reader, to enhance reading resolution in a main scanning direction, equivalent to a direction in which the image sensing elements of the CCD are arranged, the number of the image sensing elements, constituting the CCD, is increased. However, this increase results in the increase in the size of the CCD, an increase in the load in designing an optical system, and an increase in the cost of the image reader. However, if each element is miniaturized, manufacturing possibilities are limited.
Japanese published unexamined patent application No. Sho58-19081 discloses a CCD image sensor, provided with photodetectors in a first row and photodetectors in a second row, where the second row of photodetectors is offset or shifted with respect to the first row of photodetectors by an amount which is equivalent to approximately a half of the width of an individual photodetector. In the CCD image sensor, the photodetectors in the first row and the photodetectors in the second row are arranged adjacent to each other in a sub scanning direction.
However, as disclosed in the Japanese published unexamined patent application No. Sho58-19081, if a CCD image sensor, in which photodetectors in a first row are arranged closely adjacent to the photodetcctors in a second row in a sub scanning direction is used, a carriage is required to be moved by the height of the photodetector at every reading of one line for the photodetectors in the first row and the photodetectors in the second row to read the same line on a manuscript.
Generally, in an image reader, reading resolution in a sub scanning direction is determined by the time required for photodetectors in each row to read one line and the traverse speed of a carriage. Therefore, in reading at low resolution in a sub scanning direction, a carriage is moved at high speed.
However, image readers provided with a CCD image sensor in which photodetectors in a first row are arranged in close contact or in close proximity to the photodetectors in a second row in a sub scanning direction, cannot perform a precise reading operation of a manuscript. More particularly, in such an image reader, when the carriage is moved at high speed, if a manuscript is read at high resolution in a main scanning direction, and is read at low resolution in a sub scanning direction, a problem arises in that lines to be read by photodetectors in the first row and photodetectors in the second row are different which prevents a precise reading of the image. This problem arises because the arrangement of the photodetectors, in the first and second rows of photodetectors, in a sub scanning direction is not considered; that is, an interval between the photodetectors in the first and second rows is not considered. Therefore, in an image reader provided with the CCD image sensor disclosed in the Japanese published unexamined patent application No. Sho58-19081, if an image is read at low resolution in a sub scanning direction, the carriage must be moved at low speed to read an image precisely, as compared to when an image is read at high resolution, and therefore, it takes a long time to perform a read operation.